The invention relates to the field of variable gain amplifiers, and in particular to variable gain amplifiers with constant input impedance and adjustable one-pole filtering characteristics.
Variable gain amplifiers (VGA) are known in the art and are utilized in a number of applications. Traditionally, a VGA comprises an operational amplifier (op-amp) having adjustable resistances in either the feedback path (from op-amp output to op-amp input), the signal input path, or both. By adjusting the resistance in either of these paths, the feedback factor of the amplifier is adjusted, thereby providing the variable gain.
One disadvantage of the traditional implementation of a VGA is that, as the gain of the VGA is adjusted by changing the feedback factor, the closed-loop bandwidth of the VGA changes. This is due to the fact that the closed-loop bandwidth of the VGA is approximately the open-loop bandwidth, which is fixed, of the op-amp (op-amp transfer function) multiplied by the feedback factor. As the gain increases, the feedback factor decreases and, therefore, the closed-loop bandwidth decreases. Therefore, for higher gains, the bandwidth of the VGA is more limited, limiting the range of input signals applied to the VGA.
U.S. Pat. No. 5,285,169, entitled Monolithic Integrated Differential Amplifier with Digital Gain Setting, seeks to correct this disadvantage by providing an operational amplifier in which the gain-bandwidth product (open-loop bandwidth) of the op-amp is adjustable. The gain-bandwidth product is made adjustable by adjusting the channel width Wt and source current xe2x80x9citxe2x80x9d of transistors t1 and t2 in the differential stage xe2x80x9ctvxe2x80x9d of the op-amp. These variables are adjusted by activating a parallel stage xe2x80x9ctoxe2x80x9d, comprising transistors t7 and t16.
While the ""169 patent addresses the drawback of a fixed gain-bandwidth product, there are a number of disadvantages to the VGA of the ""169 patent, as well as the traditional VGA. The adjustment of the resistance in the input path causes the input impedance to change. Particularly when a VGA is utilized to implement an analog front-end system, this creates difficulties in matching the driver of the input to the input impedance.
In addition, at times it is desirable provide a one-pole filtering characteristic with the VGA. Often this is done to reduce noise in the input signal. This is typically accomplished by providing capacitance in the feedback path of the op-amp. However, when traditional VGAs that adjust the resistance in the feedback path to adjust the gain are utilized in conjunction therewith, the time constant, and implicitly, the pole location of the VGA is changed as the gain is changed.
Therefore, what is needed is a variable gain amplifier which has constant input impedance and one-pole filtering characteristics which are adjustable so as to remain constant for a change in gain.
The present invention provides for a variable gain amplifier with constant input impedance and an adjustable one-pole filtering characteristic. Preferably, a first set of n+1 parallel resistors is connected to input Vin and a second set of n+1 parallel resistors is connected to input Vip. Except for one resistor in each set, each of the resistors of the two sets is connected to its corresponding summing junction (op-amp input), or to a corresponding resistor of the other set via a switch. Switching the resistors to their corresponding summing junction or to the corresponding resistor of the other set provides for the variable gain function, where the gain is proportional to the number of resistors connected to the summing junction. However, regardless of the number of resistors switched to the summing junctions, the input impedance of the VGA remains a constant impedance of R/(n+1), where R is the resistance value of the resistors. In addition to the input resistor networks, the VGA of the present invention comprises a fixed feedback resistance, along with, preferably, a parallel capacitor array in the feedback paths of the differential op-amp to provide one-pole filtering of the input signal. Changing the value of the capacitance, by switching capacitors in the array in and out, changes the one-pole filtering characteristics without affecting the VGA gain. To maintain the filtering characteristics constant for a change in the gain, the op-amp comprises an internal compensation network for adjusting the open-loop bandwidth of the op-amp.